Shower drips while tub runs

I installed a new tub fixture. It is the two handle type with 1/2 inch copper going up to the shower and 1/2 copper going down and over to the tub spout. When I run the tub, water drips out of the shower. What might be the problem?

When you say you ran the tub spout down and over are you saying it's not just under the valve? If so this may be your problem. If you have to many fittings between the valve and the spout the added restriction is forcing some water to the shower head.